1. Field of the Invention
This invention relates, in the first place, to a method of testing the operative condition of cooling sections of a slot die for making flat films of thermoplastic material. The cooling sections are provided with lines for supplying and discharging cooling air, and the lines for supplying cooling air are connected, as required, to a central cooling air feed line. The feed line is under superatmospheric pressure, supplied through solenoid valves controlled by an electronic computer.
This invention relates also to a method of testing the operative condition of cooling sections of an annular die of a blow molding extruder for making tubular films from synthetic thermoplastics. The cooling sections are provided with lines for supplying and withdrawing cooling air, and the lines for supplying cooling air are connected to a central cooling air feed line. The feed line is under superatmospheric pressure, supplied through solenoid valves which are controlled by an electronic computer.
2. Description of the Prior Art
In the production of flat films and blow molded tubular films, it is known to provide a slot die or annular die with cooling sections, which, in response to a measured deviation of thickness of the extruded film from the average film thickness, are temporarily supplied with cooling air to ensure that the extruded film will have a uniform thickness over its width or over its periphery. In the production of flat films, die sections will be cooled when excessively thick film portions have been extruded therefrom. In blow molding tubular films, the annular extrusion die will be, at least temporarily, cooled in those sections from which film portions have been extruded which are too thin after the film has been inflated. These methods are known for a control of the production of flat and tubular films having a good average thickness distribution.
A slot die for making a flat film or an annular die for making a blown film is divided into numerous cooling sections, which must individually be supplied with cooling air when needed. Disturbances may occur, however, at individual or several cooling sections, and satisfactory operation of individual cooling sections may no longer be ensured. It was previously impossible to detect disturbances at individual cooling sections unless a failure of the automatic control was noticed. Moreover, it is usual for the machine operator to check the operation of the solenoid valves by feeling the flexible tubes which lead from respective solenoid valves to the slot die or to the film blowing head. Because the flexible supply tubes are relatively thin-walled, opening a valve will result in a slight bulge in the flexible supply tube, and that bulge can be felt. Some valves, however, may be opened rather frequently, and others may hardly be opened, so that the check, which is manually performed by the machine operator, is rather inaccurate and takes much time.